1. Field of the Invention
The present invention relates to a process for producing high purity terephthalic acid by the liquid phase oxidation of a p-dialkylbenzene such as p-xylene, more particularly, to a process of producing high purity terephthalic acid by introducing a reaction mixture containing terephthalic acid formed by continuous oxidation to a vessel other than the oxidation reactor and treating the reaction mixture therein for an extended period of time with a molecular oxygen containing gas having a low oxygen concentration.
2. Description of the Prior Art
Various processes have hitherto been proposed for producing terephthalic acid by subjecting a p-dialkylbenzene such as p-xylene to a liquid phase oxidation with molecular oxygen in the presence of a solvent such as acetic acid and an oxidation catalyst. These prior art processes are generally classified into the following groups according to the reaction mode.
One group relates to a batch system wherein the total amount of feed dialkylbenzene, solvent and catalyst is charged into a reactor prior to reaction, an oxygen containing gas is then introduced and, after the reaction is completed, the total amount of the reaction mixture is discharged from the reactor.
Another group involves a semi-continuous system wherein a feed dialkylbenzene (in some cases, together with a part of the solvent and catalyst to be used) and an oxygen containing gas are continuously introduced into a reactor containing all or the balance of the solvent and catalyst, and, after the reaction is completed, the total amount of the reaction mixture is discharged from the reactor.
The last group relates to a continuous system wherein a feed dialkylbenzene and an oxygen containing gas are continuously introduced into a reactor together with a solvent and a catalyst and then the reaction mixture is continuously or intermittently discharged from the bottom of the reactor.
Among these systems, the present invention is particularly directed to an improvement of the continuous system.
Recently, a direct polymerization process for producing a polyester from terephthalic acid by directly polymerizing high purity terephthalic acid and ethylene glycol or ethylene oxide was developed. This polymerization process is economically very advantageous over conventional processes where terephthalic acid is polymerized after it has once been esterified into dimethyl terephthalic which can be purified more easily than terephthalic acid. However, in direct polymerization high quality terephthalic acid is required as a raw material. Also, the direct production of terephthalic acid by a continuous system has high industrial advantage since the process can be performed in a simplified manner, terephthalic acid having constant quality can be produced in a stable fashion for long periods of time, and, further, the yield of product per unit volume of the reactor and per unit time is better than in the batch system and the semi-continuous system.
However, direct continuous processing has the demerit that since the amount of the reaction material discharged from the reactor without being contained a sufficient retention time in the reactor is larger than in the batch system and the semi-continuous system, the quality of the terephthalic acid produced is insufficient due to the increased contents of unreacted feed material and reaction intermediate product, e.g., reaction intermediate products such as 4-carboxybenzaldehyde, p-toluic acid, etc., and, though in very slight amounts, very active compounds in addition to terephthalic acid. If oxidizable materials such as these reaction intermediate products and active compounds are not rapidly oxidized to be converted into terephthalic acid or non-toxic materials (in this invention the reaction of completing the oxidation is called "stabilization" and the reaction vessel for performing the stabilization is called a "stabilizer"), they are converted into side reaction products such as fluorenone, diketones, etc., which reduce the quality, in particular, the color hue, of the product terephthalic acid.
Therefore, an improvement has been proposed wherein high purity terephthalic acid is obtained by completing the oxidation of the reaction intermediate products and the active materials contained in the reaction mixture after the oxidation reaction is completed. For example, Japanese Patent Publication No. 12,695/'65 discloses a process of producing high purity terephthalic acid by subjecting a reaction mixture containing terephthalic acid to further oxidation with air for about one hour at temperatures (200.degree. to 300.degree. C.) higher than the temperature in the liquid phase oxidation of p-dialkylbenzene after the oxidation reaction is completed. However, although this process is efficient in achieving complete oxidation, it is accompanied by disadvantages such as the reaction product is unavoidably colored because of the formation of by-products due to the additional treatment with oxygen at higher temperatures after the oxidation reaction is completed, and, further, there is the possibility of explosion due to the increase in the concentration of oxygen in the reaction system which may exceed the explosive limit (8% by volume oxygen concentration in the gas phase) due to the continuous introduction of air into the system over an extended period of time.
To improve the disadvantages in conventional oxidation processes, the inventors previously discovered a process of producing high purity terephthalic acid of good color hue continuously as described in U.S. Pat. No. 3,859,344 wherein a p-dialkylbenzene is continuously subjected to a liquid phase oxidation with molecular oxygen in the presence of a solvent and an oxidation catalyst in a reaction vessel, the oxidation reaction product obtained is partially discharged from the reaction vessel and introduced into a small size stabilizer having such a capacity that the amount of the discharged reaction product each time occupies 30 to 70% by volume of the contents of the stabilizer, and then the reaction product is further subjected to an oxidation treatment with air for 1 to 5 minutes at a temperature the same as or lower than that in the preceding oxidation reaction in such a range that the oxygen content in the reaction system does not exceed the explosive limit. That is, to perform the complete stabilization of the materials to be oxidized in the reaction product discharged in an oxidation reaction vessel, it is required to sufficiently contact the materials with air, but when an oxygen containing gas having a high oxygen content such as air is used for this purpose, the oxygen content in the stabilizer may exceed the explosive limit to give a possibility of explosion.
Thus, based on the discovery that it is necessary to not reduce the concentration of the materials to be oxidized in the reaction mixture to effectively perform the stabilization within an oxygen content range not exceeding the explosive limit, the inventors previously established the process of the aforesaid U.S. patent in which the reaction mixture containing the products is partially introduced into a small size stabilizer in an amount of 30 to 70% by volume of the contents of the stabilizer to bring the reaction mixture into contact with air for a short period of time (1 to 5 minutes) in the stabilizer, and then the greater part of the reaction mixture is immediately discharged from the stabilizer to repeat the same step.
According to the process of this U.S. patent, stabilization can be effectively achieved with an oxygen content in the stabilizer which does not exceed the explosive limit. However, in the process it is required to frequently repeat the steps of discharging the reaction mixture intermittently in a constant amount from an oxidation reaction vessel, introducing the reaction mixture into a small size stabilizer, bringing it into contact with air for 1 to 5 minutes with an oxygen content in the system which does not exceed the explosive limit, stopping the introduction of air, and discharging the greater part of the reaction mixture in the stabilizer. Thus, the process is complicated. In particular, since in this process the temperature and the pressure in the stabilizer change greatly due to the large volume change of the contents of the stabilizer, which makes it difficult to control the temperature and the pressure, and also the composition of the contents of the stabilizer changes frequently and periodically, the merits of continuously producing products of constant quality over a long period of time in a stable manner is not effectively utilized. Furthermore, in this process the introduction of the oxygen containing gas is temporarily stopped, which results in readily causing precipitation of the product at the bottom of the stabilizer, etc.
Therefore, it is most desirable to establish a continuous process of producing terephthalic acid of constant high quality for extended periods of time by effectively achieving stabilization with an oxygen content in the reaction system which does not exceed the explosive limit while introducing an oxygen containing gas continuously into the stabilizer without interrupting the introduction of the gas.